Sheet pin hole detector



W. E. COLEMAN l' AL SHEET FIN HOLE DETECTOR Feb. 28, 1956 Feb. 28, 1956 w. E. COLEMAN ET Al.

SHEET PIN HOLE DETECTOR 2 Sheets-Sheet 2 F'iled Feb. 19. 1952 f m M wh e E m n M M W A U B C D E United States Patent-*lift O 1 2,736,431 SHEET PIN HOLE DETECTOR William E. Coleman and George H. Rendel, Pittsburgh,

Pa., assignors to United States Steel Corporation, a corporation of' New Jersey Application February 19, 1952, Serial No. 272,471 15 Claims. (Cl. NP9-lib This invention relates to a pin hole detector and more particularly to a detector for detecting and classifying thin sheets of material such as steel for tin plate. In the rolling of strip for use in making tin plate small perforations known as pin holes are formed in the strip. At present the strip is inspected for these pin holes by means of apparatus such as shown in Chamberlin et al. Patent No. 2,229,638. The apparatus therein described was designed and is suitable for inspecting strip, but it and other such apparatus are not suitable for detecting pin holes in individual sheets. Therefore, the common method of inspecting individual sheets is by visual inspection and manual handling of each sheet. Such method is slow and expensive.

It is therefore an object of our invention to provide a pin hole detector for detecting pin holes and classifying thin sheets of material.

This and other objects will be more apparent after referring to the following specification and attached drawings, in which:

Figure l is a schematic drawing of the inspection line with the detecting and classifying control incorporated therein;

Figure 2 is a sectional elevation taken on the line lI-Il of Figure l; and

Figure 3 is a view showing the voltage wave forms in part of the apparatus.

Referring more particularly to the drawings, the reference numeral 2 indicates a pile of sheets which are to be inspected. The sheets S are fed from the pile 2 by means of a feeder 4 such as the well known Dexter feeder to a motor driven conveyor belt 6. The' sheets S pass from the conveyor 6 to a second conveyor 8. The prime sheets pass over a defiector 10 to another motor driven belt .conveyor 12 and then to the prime sheet pile i4. The defective sheets pass from the conveyor 8 below the deliector 10 to a belt conveyor 16 and then to a salvage pile 18. The conveyors 6 and 8 are spaced apart a distance less than the length of the lshortest sheet to' be inspected. Side guides 20 for the sheets are provided between the conveyors 6 and 8'. A source of light 22 is provided above the path of travel of the sheets S between the conveyors 6 and S and a second source or" light 24 is provided on the same side of the path of travel of the sheets S toward the conveyor 8 from the light source 22. The light sources 22 and 24 provide light beams 26 and 28 respectively. A group of photoelectric cells 30 are located directly below the light source 22 and below the path of travel of the Sheets S. A pair of photocells 32 connected in series are also located below the light source 22. The photocells 30 and 32 are mounted in a common housing '34. The photocells 30 are connected to an amplifier 36 which is of the type that puts out a negative pulse followed by a positive pulse each time light strikes one or more yof the photocells 30. Thus a negative and positive pulse will occur at the output of the amplifier 36 each time a hole is detected in the sheet and each time the trailing end of the'sheet exposes the photocells 30 to lthe light. A small hole will provide a very `small input pulse to the amplifier whereas the great amount of light which strikes the photocells 30 when the trailing rend of the .sheet passes the light beam 26 results in Va very 'large input pulse to the amplifier 36. The amplifier 36 is designed to limit the output of the large impulse to a value not appreciably greater than the impulse from a very small hole. As shown, the amplifier 36 consists of a D. C. amplifier 3S and A. C. amplifiers 4t) and 42. The first stage amplifier 38 is a conventional electron tube amplifier directly coupled to the photocells 30. The amplifier Sti is coupled to the second stage amplifier 40 through a differentiator circuit consisting of a capacitor 44 and res-istor 46. The amplifier 40 is a single electron tube amplifier operating with zero grid bias and is coupled to the third stage amplifier 42 through a normal long time constant coupling circuit consisting of a capacitor 48 and resistor Sti. Amplifier 42 is also a single electron tube amplifier and is biased approximately at cut off. The output of amplifier 42 is connected to a resistor 52 through a differentiating circuit consisting of a capacitor 54 and resistor 56. Power for the amplifiers 38, 46 and 42 is supplied by means of a D. C. power source 58. The voltage wave forms at points A, B, C, D, E and F are shown in Figure 3. These wave forms are drawn to show the voltage variations in the circuit when one sheet containing a hole approximately in the center passes through light beam 26. When the leading end of the sheet crosses light beam 26 the input voltage to amplifier 3S drops as a result of light being removed from photocells 3i? as shown in wave form A. When the hole passes light beam 26 a small positive pulse occurs and when the trailing edge clears the light beam 26, the voltage again rises. These voltage variations are amplified and reversed in amplifier 33 and the output at point B is shown in wave form B. This voltage variation is impressed on the differentiator circuit consisting of capacitor 44 and resistor 46 and results in the wave form C at point C. It will be noted that the voltage at the leading end of the sheet will fall gradually due to the charging of condenser 44. In like manner the voltage at the trailing end of the sheet will rise gradually due to the condenser discharging. This Voltage is applied to amplifier 40 which as previously mentioned is operated at zero grid bias so that the positive pulses are limited. This permits only a small part of the initial positive pulse to pass as indicated by the dotted line in wave form C and gives an output voltage from amplifier 40 as shown in wave form D. Since the trailing end pulse is so large 'that the amplification thereof is beyond the limit of the amplifier, it is not amplified as much as the hole pulse but is somewhat widened as shown. The output voltage of amplifier 4th is applied to amplifier 42 which is adjusted to operate with sufficient grid bias to approximately cut off its plate current when no input pulse is bei-ng applied. Thus the small remaining negative pulse of the leading edge in wave form D is not passed by ampliier 42. However, the positive pulses for the hole vand the trailing end are amplified and the output thereof is that shown in wave form E. It will be noted that the hole pulse is now approximately of the same amplitude as that of the trailing end of the sheet. The output of amplifier 42 is applied to the differentiator circuit consisting of capacitor 54 and resistance 56. This provides an output voltage as shown in wave form F. This voltage is applied through resistor 52 to the grid 60G of a gas tube 6i) and to the plate 621J of a vacuum tube 62. The voltage produced at the grid 62G as a result of light striking the two photocells 32 is shown in wave form G. The result of the voltage at point F and the voltage at point G is shown as wave form H at the plate 62P.

The photocells 32 are connected to voltage amplifier Y64 which in turn is connected to a tube 66. The tube -66 may be part of the amplifier 64 but is shownvseparately in order to clarify the description. 'LightA Striking' both Ynormally highly conducting. `in the connection between the D. C. power source 76 and Q ,r of photocells 32 will cause a drop in voltage on grid 66G. The use of two photocells is not essential, but two are used instead of one to prevent a large hole over one photocell from blanking out the signal in the manner described below. The amplifier 64 and tube 66 are supplied with power from a D. C. power source 68 which has its positive side grounded. Tube 66 is normally conducting, that is, current exists between its plate and cathode and the plate 66P is at a potential below ground because of the drop in resistor 70 connected in the circuit. The plate 66P is directly connected to the grid 62G. The tube 60 is normally held non-conducting by a positive bias on its cathode 60C which is obtained by the voltage divider consisting of resistors 72 and 74 which obtain power from a D. C. power source 7 6. When the tube 60 starts to conduct it will continue to conduct until the plate to cathode potential is reduced to zero. A resistance 78 is connected between the positive terminal of D. C..power source 76 and the plate 691. The resistance 78 is connected to the terminals of a voltage amplier 80 which is connected to a photoelectric cell 82 which receives light from the light source 24. When there is a drop in voltage across the resistor 74 it is transmitted through a capacitor 84 and appears as a negative pulse across resistor 86 and on grid 88G of a vacuum tube 88 which operates with no grid bias so that it is A resistor 90 is connected the plate 88P. The tube 88 will not be affected by a positive pulse but the negative pulse which occurs when light strikes photocell 82 will reduce the current in the tube 88 and resistor 90 to permit the potential of plate 881 to rise, thus providing a short positive pulse which is transmitted to the pulse-lengthener 92. The pulselengthener 92 may be a conventional one-shot cathode coupled multivibrator triggered by a. triode ampliiier such as shown in the copending application to Coleman, Serial No. 133,343, now abandoned, filed December 16, 1949 and its continuation-in-part application, now Patent No. 2,655,620, dated October 13, 1953. Pulse-lengthener 92 provides an output pulse of desired duration which controls a gas tube switch 94 or similar device, which in turn applies alternating current from A. C. power source 96 to a solenoid 98 which is connected to operate the gate 10.

The operation of the device is as follows:

If there is no sheet passing under the light 22, the light rays 26 will fall on all of tubes 38 and 32. Light falling on tubes 30 will cause the amplifier 36 to put out a negative pulse followed by a positive pulse. Light falling on the tubes 32 results in a pulse which causes a voltage drop on grid 66G of suiiicient amplitude to cut oi current flow through tube 66. Plate 66T will then be at ground potential. Since grid 621C: is connected directly to plate 66P, this will raise the potential of grid 62G from 'a value well below ground, up to ground potential, thus permitting tube 62 to conduct heavily for the period in which light is falling on photo tubes 32. Thus tube 62 acts as a very low resistance during this time and short circuits the positive output pulse of ampliler 36. Since this positive pulse is necessary to cause tube 60 to conduct, the tube 60 will not conduct at this time. Since the negative pulses from amplifier 36 have no effect on tube 60, they need not be considered. It will be seen by reference to Figure 3 that even though the voltage at grid 62G takes an appreciable time to reach its maximum value, it is still in time to blank out the positive pulse from amplifier 36. This is due to the fact that the positive pulse F is delayed so that even though the blanking photocells 32 receive light at the same time as the main photocells 30, the main pulse is delayed for a fraction of a second. This is important since a slight amount of light leakage around the tail end of the sheet may permit the main pulse to start early and reach an appreciable magnitude before the blanking takes effect. When a perfect sheet is lifted from the pile 2 by means of the feeder 4 l and passes through the light rays 26, it will prevent light falling on tubes 30 and 32 and the sheet will pass over the decctor 10 into prime piler 14. When an imperfect sheet passes through the light rays 26, there will be light impressed on one of the tubes 30, causing the impulse F". However, since the tubes 32 are spaced apart the light through the pin hole will fall on either one or none of the tubes 32. Thus there will be no impulse from the tube 62 to blank out the positive pulse F" and the tube 60 will be tired. While under some circumstances the gate 10 might be actuated directly from the pulse caused by the hole, it is desirable to make the classification timing independent of the position of the hole in the sheet. The photocell 82, in conjunction with other parts of the apparatus shown in Figure 1, is used to refer the hole to the trailing end of the sheet and operates in the following manner:

The gas tube 60 will conduct until the trailing end of the sheet containing the hole passes through light beam 28, thus permitting light to strike photocell 82. This causes a pulse which is amplified in amplifier and transmitted as a negative pulse through capacitor 83 to plate 60P. This negative pulse is of suicient amplitude to drive the potential of plate 60P below the potential of cathode 60C momentarily and thus extinguish the tube 60 and reduce the potential drop across resistor 74. This drop in voltage across resistor 74 is transmitted through the capacitor 84 and appears as a negative pulse across resistor 86 and on grid SSG. Since tube 88 operates with no grid bias, it is normally highly conducting and the positive pulse occurring on its grid when tube 60 lires has no appreciable effect but the negative pulse reduces the current in tube 88 and resistor 90 and permits the potential of plate 881 to rise, thus providing a short positive pulse into pulse lengthener 92 which provides an output pulse to operate the gas tube switch 94 and apply power to operate solenoid 98. This raises the deector 10 to cause the imperfect sheet to pass over the conveyor 16 into pile 18. As soon as the pulse from pulse lengthener 92 is extinguished the gate 10 will return to its normal position. y While one embodiment of our invention has been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. Apparatus for detecting pin holes in moving sheets comprising means for moving the sheets along a normal path of movement, a source of light on one side of the normal path of movement of said sheets, a photosensitive device on the other side of the normal path of movement constructed and arranged for detecting pin holes, means operable by light falling on said photosensitive device for directing sheets with a pin hole therein out of the normal path of movement of said sheets, a second photosensitive device operable by light from said rst light source located adjacent the Iirst photosensitive device constructed and arranged for preventing operation of said means when there is no sheet between the light and said photosensitive devices, a second source of light on the exit side of said rst source, a photosensitive device on the other side of the normal path of movement operable by said second light source, and means operable by light falling on said last named photosensitive device for controlling start of operation of said first named means.

2. Apparatus for detecting pin holes in moving sheets comprising a conveyor system for said sheets, a prime sheet piler, a defective sheet piler, said sheets normally traveling to said prime sheet piler, a dei'lector for directing sheets to the defective sheet piler, a source of light on one side of the path of movement of said sheets, a photosensitive device on the other side of the path of movement constructed and arranged for detecting pin holes, means operable by light falling on said photosensitive device for operating said deflector to direct sheets with pin holes 5 therein to the defective sheet piler, and a second photosensitive device operable by light from said nist light source located adjacent the first photosensitive device constfucted and arranged for preventing operation of said means when there is no sheet between the light and said photosensitive devices, a second source of light on the exit side of said first source, a photosensitive device on the other side of the path of movement operable by said second light source, and means operable by light falling on said last named photosensitive device for controlling start of operation of said first named means.

3. Apparatus for detecting pin holes in moving sheets comprising means for moving the sheets along a normal path of movement, a source of light on one side of the normal path of movement of said sheets, a photosensitive device on the other side of the normal path of movement for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, means connected to said photosensitive device for amplifying an impulse therefrom and changing it to a positive and negativ'e pulse, an electronic tube becoming conductive by a positive impulse from said'last named means, means operable by said electronic tube rfor directing sheets with a pin hole therein out of the normal path of movement of said sheets, a second photosensitive device adjacent the first photosensitive device, a second electronic tube operable by an impulse from said second photosensitive device, and means connecting the plate of the second tube to the output of said amplifying means to shor't the output of said amplifying mea-ns 'and prevent operation of said first tube when there is-no sheet between the light and said photosensitive device.

4. Apparatus for detecting pin holes in moving Sheets comprising means for moving kthe sheets along a normal path of movement, a source of ylight 'on 'one side 'of the normal path of movement of said sheets, a photosensitive device on the'other side ofthe normal path of movement for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, a voltage -ampliiier connected to said photosensitive device, an electronic tube becoming conductive by an impulse'from said voltage amplifier, means operable by said electronic -tube for directing sheets with a pin hole therein out of the normal path of movement of said sheets, means for preventing operation of said tube when there is no sheet between the light and said photosensitive device, a second source of light on the eXit side of said first source, a photosensitive device on the other side of the path of movement operable by said second light source, and lmeans operable by light falling on said last named photosensitive device 7 for controlling start of operation 'of vsaid first named means.

5. Apparatus for detecting pinholes in moving sheets ycomprising means for moving the sheets 'along a normal path of movement, a source of light on one side of the normal path of movement of said sheets, a photosensitive device on the other side of the normal path of movement for detecting pin holes, said lphotosensitive device generating an electric impulse when .light falls thereon, a voltage amplifier connected to said first named photosensitive device, an electronic tube becoming conductive by an impulse from said voltage amplifier, means 'operable by said electronic tube for directing sheets with a pin hole therein out of the normal path of movement k of said sheets, a second photosensitive device adjacent the first photosensitive device, a second electronic tube operable by an impulse from said second photosensitive device for preventing operation of said first tube when there is no sheet between the light and said photosensitive device, a second source of light on 'the vexit Aside of said first source, a photosensitive device on the other side of the normal path of movement operable by said second light source, and

means operable by light vfalling on said last named photosensitive delvice for-controlling start of operation of said fi'rs't named means.`

6. Apparatus for detectiug'pin holes in moving' sheets comprising means for moving the sheets along a normal path of movement, a source of ylight on one side of the normal path of movement of said sheets, a photosensitive device on the other side of the normal path of movement for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, means connected to said photosensitive device for amplifying an impulse therefrom and changing it to a positive and negative pulse, an electronic tube becoming conductive by a positive impulse from said last named means, means operable by said electronic tube for directing sheets with a pin hole therein out of the normal path of movement of said sheets, a second photosensitive device adjacent the first photosensitive device, a second electronic tube oper'- able by an impulse from said second photosensitive device, means connecting the plate of the second tube to the output of said amplifying means to short the output of said amplifying means and prevent operation of said first tube when there is no sheet between the light and said photosensitive device, a second source of light on the exit side of said first source, a photosensitive device on the other side of the normal path of movement operable by said second light source, and means operable by light falling on said last named photosensitive device for ycontrolling start of operation of said second named means.

7. Apparatus for detecting pin holes in moving sheets comprising a conveying system for said sheets, a prime sheet piler, a defective sheet piler, said sheets normally traveling to said prime sheet piler, a defiector for directing sheets to the defective sheet piler, a source of light on one side of the path of movement of said sheets, a photosensitive device on the other side of the path of movement for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, mea-ns connected to said photosensitive device for amplifying an impulse therefrom and changing it to a positive and negative puise, an electronic tube operable by a positive impulse from said last named means, means operable by said electronic tube for operating said deflector to direct sheets with pin holes therein to the defective sheet piler, a second photosensitive device adjacent the first photosensitive device, a second electronic tube operable by an impulse from said second photosensitive device, and means connecting the plate of the second tube to the output of said amplifying means to short the output of said amplifying means and prevent operation of said first tube when there is no sheet between the light and said photosensitive device.

8. Apparatus for detecting lp'iri holes -in 'moving sheets comprising a conveying system 'for said sheets, 'a -prime sheet piler, a defective sheet piler, said sheets normally traveling to said prime sheet piler, a deliector' for vdirecting sheets to the defective sheet piler, a source of light-'on one side of the path of movement of said sheets, aphotosensitive device on the other side of the path of Vmovement for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, a voltage amplifier connected to said photosensitive `device, an electronic tube operable byY an impulse from'v said voltage amplifier, means operable by said electronic tube for operating said deliector to direct sheets with pin 4holes therein to the defective sheet piler, means for preventing operation of said tube when there is no sheet between the light and said photosensitive device, a second 'source of light on the exit side of said'first source, aphotosensitive device on the other side of the path of movement operable by said second light source, vand means operable by light falling on said last `named photosensitive device for controlling start of operation *of said first fnanied means.

9; Apparatus vfor detecting pin holes in moving sheets comprising a conveying system for said sheets, aprirne `sheet piler, a defective sheet piler, said sheets' normally traveling to said prime sheet piler, a detiector for direct.-4

atea-131 ing sets to the defective shet piler, a source of light on onel side of the path of movement of said sheets, a photosensitive device on the other side of the path of movement constructed and arranged for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, a voltage amplifier connected to said photosensitive device, an electronic tube operable by an impulse from said voltage amplifier, means operable by said electronic tube for operating said deflector to direct sheets with pin holes therein to the defective sheet piler, a second photosensitive device adjacent the first photosensitive device, a second electronic tube operable by an impulse from said second photosensitive device for preventing operation of said first tube when there is no sheet between the light and said photosensitive device, a second source of light on the exit side of said first source, a photosensitive device on the other side of the path of movement operable by said second light source, and means operable by light falling on said last named photosensitive device for controlling start of operation of said first named means.

10. Apparatus for detecting pin holes in moving sheets comprising a conveying system for said sheets, a prime sheet piler, a defective sheet piler, said sheets normally traveling to said prime sheet piler, a defiector for directing sheets to the defective sheet piler, a source of light on one side of the path of movement of said sheets, a photosensitive device on the other side of the path of movement for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, means connected to first named photosensitive device for amplifying an impulse therefrom and changing it to a positive and negative pulse, an electronic tube operable by a positive impulse from said last named means, means operable by said electronic tube for operating said deliector to direct sheets with pin holes therein to the defective ysheet piler, a second photosensitive device adjacent the first photosensitive device, a second electronic tube operable by an impulse from said second photosensitive device, means connecting the plate of the second tube to the output of said amplifying means to short the output of said amplifying means and prevent operation of said first tube when there is no sheet between the light and said photosensitive device, a second source of light on the exit side of said first source, a photosensitive device on the other side of the path of movement operable by said second light source, and means operable by light falling on said last named photosensitive device for controlling start of operation of said second named means.

11. Apparatus for detecting pin holes in moving sheets comprising means for moving the sheets along a normal path of movement, a source of light on one side of the normalv path of movement of said sheets, a photosensitive device on the other side of the normal path of movement constructed and arranged for detecting pin holes, means operable by light falling on said photosensitive device for directing sheets with a pin hole therein out of the normal path of movement of said sheets, a second photosensitive device operable by light from said first light source located adjacent the first photosensitive device constructed and arranged for preventing operation of said means when there is no sheet between the light and said photosensitive devices, and means operable by an end of the sheet being inspected for controlling start of operation of said first named means.

12. Apparatus for detecting pin holes in moving sheets comprising a conveying system for said sheets, a prime sheet piler, a defective sheet piler, said sheets normally traveling to said prime sheet piler, a detiector for directing sheets to the defective sheet piler, a source of light on one side of the path of movement of said sheets, a photosensitive device on the other side of the path of movement constructed and arranged for detecting pin holes, means operable by light falling on said photosensitive device for operating said deector to direct sheets with pin holes therein to the defective sheet piler, a second photosensitive device operable by light from said first light source located adjacent the first photosensitive device constructed and arranged for preventing operation of said means when there is no sheet between the light and said photosensitivedevices, and means operable by an end of the sheet being inspected for controlling start of operation of said first named means.

13. Apparatus for detecting pin holes in moving sheets comprising means for moving the sheets along a normal path of movement, a source of light on one side of the normal path of movement of said sheets, a photosensitive device on the other side of the normal path of movement constructed and arranged for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, a voltage amplifier connected to said photosensitive device, an electronic tube operable by an impulse from said voltage amplifier, means operable by said electronic tube for directing sheets with a pin hole therein out of the normal path of movement of said sheets, means for preventing operation of said tube when there is no sheet between the light and said photosensitive device, and means operable by an end of the sheet being inspected for controlling start of operation of said first named means.

14. Apparatus for detecting pin holes in moving sheets comprising means for moving the sheets along a normal path of movement, a source of light on one side of the normal path of movement of said sheets, a photosensitive device on the other side of the normal path of movement constructed and arranged for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, a voltage amplifier connected to said first named photosensitive device, an electronic tube operable by an impulse from said voltage amplifier, means operable by said electronic tube for directing sheets with a pin hole therein out of the normal path of movement of said sheets, a second photosensitive device adjacent the first photosensitive device, a second electronic tube operable by an impulse from said second photosensitive device for preventing operation of said first tube when there is no sheet between the light and said photosensitive device, and means operable by an end of the sheet being inspected for controlling start of operation of said first named means.

15. Apparatus for detecting pin holes in moving sheets comprising means for moving the sheets along a normal path of movement, asource of light on one side of the normal path of movement of said sheets, a photosensitive device on the other side of the normal path of movement for detecting pin holes, said photosensitive device generating an electric impulse when light falls thereon, an electronic tube operable by an impulse from said photosensitive device, means operable by said electronic tube for directing sheets with a pin hole therein out of the normal path of movement of said sheets, a second photosensitive device operable by light from said first light source located adjacent the first photosensitive device, means for connecting the output of said second photosensitive device to said tube for preventing operation thereof when there is no sheet between the light and said second photosensitive device, means for delaying the action of the impulse from said first photosensitive device on said tube, and means operable by an end of the sheet being inspected for controlling the start of operation of said first named means.

References Cited in the file of this patent UNITED STATES PATENTS 2,229,638 Chamberlin Jan. 28, 1941 2,395,482 Hurley, Jr. Feb. 26, 1946 2,433,685 Dowell Dec. 30, 1947 2,563,213 Coleman Aug. 7, 1951 2,570,288 Todd Oct. 9, 1951 

1. APPARATUS FOR DETECTING PIN HOLES IN MOVING SHEETS COMPRISING MEANS FOR MOVING THE SHEETS ALONG A NORMAL PATH OF MOVEMENT, A SOURCE OF LIGHT ON ONE SIDE OF THE NORMAL PATH OF MOVEMENT OF SAID SHEETS, A PHOTOSENSITIVE DEVICE ON THE OTHER SIDE OF THE NORMAL PATH OF MOVEMENT CONSTRUCTED AND ARRANGED FOR DETECTING PIN HOLES, MEANS OPERABLE BY LIGHT FALLING ON SAID PHOTOSENSITIVE DEVICE FOR DIRECTING SHEETS WITH A PIN HOLE THEREIN OUT OF THE NORMAL PATH OF MOVEMENT OF SAID SHEETS, A SECOND PHOTOSENSITIVE DEVICE OPERABLE BY LIGHT FROM SAID FIRST LIGHT SOURCE LO- 